The prior art describes various devices and processes allowing separation of feeds in a simulated moving bed. U.S. Pat. No. 2,985,589; U.S. Pat. No. 3,214,247; U.S. Pat. No. 3,268,605; U.S. Pat. No. 3,592,612; U.S. Pat. No. 4,614,204; U.S. Pat. No. 4,378,292; U.S. Pat. No. 5,200,075; U.S. Pat. No. 5,316,821 and patent applications EP-0,769,316; FR-2,772,634 and WO-95/03,867 can be mentioned.
In general, a simulated moving bed comprises at least three chromatographic zones, advantageously four or five, each zone consisting of at least one bed or column section.
Between two zones, there is either an injection point for a feed to be fractionated or an injection point for an eluent or desorbent, or a point allowing to draw off an extract between the eluent injection point and the feed injection point situated downstream (in relation to the direction of circulation of the eluent), or a raffinate draw-off point between each mixture injection point and the eluent injection point situated downstream in relation to the direction of circulation of the eluent.
All the beds or column sections form a closed loop comprising at least one flow-controlled pump allowing to recycle the main fluid, for example between the first and the last section.
During the separation process, the injection and draw-off points are generally offset in the same direction (downstream or upstream, still in relation to the direction of circulation of the main fluid) by at least one section or column. This is the basis of the principle of simulated moving bed operation.
During this process, it is important that distribution of the fluid on each adsorbent bed is performed as uniformly and homogeneously as possible.
Distribution on each bed requires collection of the stream coming from the previous bed (main fluid circulating in the direction of the principal axis of the column), the possibility of injecting an auxiliary fluid or secondary fluid therein while mixing these two fluids as thoroughly as possible, or the possibility of drawing off part of the fluid collected, of extracting it in order to send it out of the device and of redistributing a fluid onto the next bed.
The whole of the main fluid or stream can therefore either transit through the adsorber according to the pattern described in U.S. Pat. No. 2,985,589, or a great part or all of this stream can flow out to the outside according to a process described in U.S. Pat. No. 5,200,075.
Another solution consists, as described in patent application FR-2,772,634, in transiting the major part of the main fluid towards the inside and a minor part of this stream towards the outside, typically 2 to 20% of this stream. An advantage of such a system is that the secondary fluid injection and extraction circuits permanently have substantially the same composition. Two distribution plates are connected by an external circuit commonly known as synchronous bypass circuit. The function of this circuit is notably to circulate the minor part of the stream drawn off thus providing an identical composition. The secondary fluid extraction on-off valves and a nonreturn valve are connected to the bypass circuit. The circuit can optionally be equipped with an on-off valve or with a control valve allowing to perform injections and extractions on a single plate.
Continuous flushing of the distribution spiders of the distribution plates of the simulated moving bed separation units can be performed in two ways:
1) when each plate is equipped with at least two independent distribution networks (D1 and D2), network D1 of plate P is for example communicated with network D2 of plate P+1, and network D1 of plate P+1 is communicated with network D2 of plate P+2, so as to have a permanent fluid circulation in all the distribution networks on each distribution plate, and, for each plate, a diverted fluid stream flows from a distribution network to the main fluid and a second diverted fluid stream, substantially similar, flows from the main fluid to the second distribution network. The driving force of these flows is provided by the pressure drop caused by the main fluid flowing in the porous granular medium located between two successive distribution plates;
2) when each distribution plate is equipped with only one distribution network, the bypass circuits can be provided only every second bed, for example from plate P to plate P+1, then from plate P+2 to plate P+3. In fact, if a bypass line connected plates P+1 and P+2, a circulation parallel to the adsorber from the top bed to the bottom bed would be obtained. The drawback of establishing a bypass circuit every second bed only is that the internal flow rates would vary from one bed to the other: the beds comprising a bypass circuit would have a flow rate D whereas the beds without a bypass circuit would have a flow rate D+b.